Forming galaxies with MOND

Abstract

Beginning with a simple model for the growth of structure, I consider the dissipationless evolution of a MOND-dominated region in an expanding universe by means of a spherically symmetric N-body code. I demonstrate that the final virialized objects resemble elliptical galaxies with well-defined relationships between the mass, radius and velocity dispersion. These calculations suggest that, in the context of MOND, massive elliptical galaxies may be formed early (z ≥ 10) as a result of monolithic dissipationless collapse. Then I reconsider the classic argument that a galaxy of stars results from cooling and fragmentation of a gas cloud on a time-scale shorter than that of dynamical collapse. Qualitatively, the results are similar to that of the traditional picture; moreover, the existence, in MOND, of a density–temperature relation for virialized, near isothermal objects as well as a mass–temperature relation implies that there is a definite limit to the mass of a gas cloud where this condition can be met – an upper limit corresponding to that of presently observed massive galaxies.